What is hymn to shiva?

The Hymn to Shiva are quite a few, however most popular of these are following.

First one is the Shiva mantra 'Aum Namaḥ Śivāya', Its short and easy to remember and hence is used as a prayer for Lord Shiva by most of the Hindus.

Second one is a bit long, it's titled as the Shiv Tandav Stotram. It is a prayer written by the Rakshasa King Ravana in the praise of Lord Shiva. You can find a youtube link to the Tandava Stotra in related links.

Even though 'Aum Namaḥ Śivāya' is a short mantra and cannot be sung as a prayer however it still fulfills the specifications of a hymn and can be considered as a hymn.